Window Box Volume Calculator

Measure your window box once, then calculate instantly. Compare liters, gallons, and bag counts quickly. Grow healthier plants with the right soil volume always.

Enter window box details
Use inside dimensions for soil space, not outer walls.
Front-to-back direction stays as width.
Opening width at the top edge.
Soil width at the base.
Radius equals planter depth for a true half-round.
Optional: gravel or drainage mat thickness.
Leave space for watering and mulch.
Typical potting mix: ~200–500 kg/m³.
Reset
Formula used
Rectangular window box
Volume = Length × Width × Effective Depth
Effective Depth = Depth − Drainage Thickness
Tapered sides (trapezoid)
Average Width = (Top Width + Bottom Width) ÷ 2
Volume = Length × Average Width × Effective Depth
Half-round (half-cylinder)
Cross‑section Area = (π × r²) ÷ 2
Volume = Area × Length
If a drainage layer is added, r is reduced approximately.
Fill adjustment
Adjusted Volume = Raw Volume × (Fill % ÷ 100)
How to use this calculator
  1. Measure the inside length, width, and depth of your planter.
  2. Select the shape that best matches your window box.
  3. Enter a drainage layer thickness if you use one.
  4. Set a fill percentage to leave watering space at the top.
  5. Use the total liters to plan soil bags and mixing needs.
  6. Download CSV for records, or PDF for a quick printout.
Example data table
Shape Dimensions Boxes Drainage Fill % Estimated total soil
Rectangular 24 in × 8 in × 7 in 2 0.5 in 90% ~40.1 L
Tapered 30 in, top 10 in, bottom 8 in, depth 8 in 1 1 in 85% ~26.1 L
Half-round 24 in length, 4 in radius 1 0.5 in 90% ~11.0 L
Examples are estimates; real planters vary by wall thickness and taper.

Why accurate volume matters

Accurate window box volume prevents underfilling that dries roots and overfilling that wastes mix. A reliable estimate helps you buy the right amount, blend amendments consistently, and keep plant spacing uniform. When each box receives the same soil depth, moisture and nutrients behave more predictably across your display. It also supports repeatable seasonal refreshes, because you can replicate last year’s volume even after trimming roots or changing liners quickly and confidently each time.

Choosing a shape model

Many planters look rectangular, but interior tapers and rounded fronts change capacity. This calculator offers rectangular, tapered, half-round, and custom-liter modes so you can match common profiles. Use inside measurements where soil actually sits. If your box flares outward, the tapered option uses the average width for a practical approximation. For irregular boxes, measure at several points and average them; small errors compound when you fill multiple planters at once.

Drainage and fill allowance

Drainage layers improve airflow and reduce waterlogging, yet they reduce soil space. Enter drainage thickness to subtract it from effective depth, or from radius in the half-round estimate. Fill percentage accounts for the headspace you leave for watering and mulch. Many gardeners target 85–95% fill to limit spillover during irrigation.

Converting volume to soil bags

Suppliers sell potting mix by liters, cubic feet, or bag size, so conversions matter at purchase time. The calculator reports liters, cubic feet, and gallons for quick comparison, then estimates 20 L, 25 L, 40 L, and 50 L bag counts. Rounding up is intentional, because settling and root growth usually demand a little extra. Record your chosen bag combination in CSV so restocking is easier during peak weekends.

Weight planning and safety

Filled boxes can be heavy on railings and brackets, especially on upper floors. By applying an adjustable soil bulk density, the tool estimates total weight in kilograms and pounds. Use the weight estimate to choose hardware, confirm mounting surfaces, and decide whether to lighten mixes with perlite or coco coir. If rain is frequent, heavier saturated soil loads can exceed dry estimates significantly.

FAQs
1) How should I measure my window box?

Measure the inside length, inside width, and soil depth with a ruler or tape. Avoid outer wall dimensions, because wall thickness reduces soil space. For tapered boxes, measure top and bottom widths.

2) What fill percentage is best for most plants?

Most window boxes perform well around 85–95% fill. This leaves room for watering, mulch, and settling. Use a lower fill if you top-dress often, and a higher fill for shallow-rooted annuals.

3) Do I need to add a drainage layer?

If your planter drains freely, a drainage layer is optional. If drainage is limited or you use a liner, a thin layer can help prevent soggy roots. Enter its thickness so the soil volume reflects the reduced depth.

4) Why is drainage applied to radius in half-round mode?

Half-round volume depends on the semicircle radius. A drainage layer occupies the bottom portion, so the soil cross-section shrinks. Reducing radius is a practical approximation that keeps estimates reasonable for common half-cylinder planters.

5) What soil density value should I enter?

Light potting mixes often range from 200 to 500 kg/m³, depending on moisture and ingredients. Use the default if you are unsure. Increase density for wetter, heavier blends and decrease it for very airy mixes.

6) How accurate are the bag estimates?

Bag counts are rounded up to avoid shortages. Brands vary by compaction and labeled volume, and soil can settle after watering. Treat the estimate as a planning tool, then adjust based on your preferred mix and handling.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.